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tion software systematic failures failure reliability testing and derived requirements field failure monitoring and corrective actions spare parts stocking availability control technical documentation caution and warning analysis data and information acquisition organisation creation of a general reliability development hazard log and fracas system chaos engineering effective reliability engineering requires understanding of the basics of failure mechanisms for which experience broad engineering skills and good knowledge from many different special fields of engineering are required 12 for example tribology stress mechanics fracture mechanics fatigue thermal engineering fluid mechanics shock loading engineering electrical engineering chemical engineering e g corrosion material science definitions edit reliability may be defined in the following ways the idea that an item is fit for a purpose the capacity of a designed produced or maintained item to perform as required the capacity of a population of designed produced or maintained items to perform as required the resistance to failure of an item the probability of an item to perform a required function under stated conditions the durability of an object basics of a reliability assessment edit many engineering techniques are used in reliability risk assessments such as reliability block diagrams hazard analysis failure mode and effects analysis fmea 13 fault tree analysis fta reliability centered maintenance probabilistic load and material stress and wear calculations probabilistic fatigue and creep analysis human error analysis manufacturing defect analysis reliability testing etc these analyses must be done properly and with much attention to detail to be effective because of the large number of reliability techniques their expense and the varying degrees of reliability required for different situations most projects develop a reliability program plan to specify the reliability tasks statement of work sow requirements that will be performed for that specific system consistent with the creation of safety cases for example per arp4761 the goal of reliability assessments is to provide a robust set of qualitative and quantitative evidence that the use of a component or system will not be associated with unacceptable risk the basic steps to take 14 are to thoroughly identify relevant unreliability hazards e g potential conditions events human errors failure modes interactions failure mechanisms and root causes by specific analysis or tests assess the associated system risk by specific analysis or testing propose mitigation e g requirements design changes detection logic maintenance and training by which the risks may be lowered and controlled at an acceptable level determine the best mitigation and get agreement on final acceptable risk levels possibly based on cost benefit analysis the risk here is the combination of probability and severity of the failure incident scenario occurring the severity can be looked at from a system safety or a system availability point of view reliability for safety can be thought of as a very different focus from reliability for system availability availability and safety can exist in dynamic tension as keeping a system too available can be unsafe forcing an engineering system into a safe state too quickly can force false alarms that impede the availability of the system in a de minimis definition the severity of failures includes the cost of spare parts man hours logistics damage secondary failures and downtime of machines which may cause production loss a more complete definition of failure also can mean injury dismemberment and death of people within the system witness mine accidents industrial accidents space shuttle failures and the same to innocent bystanders witness the citizenry of cities like bhopal love canal chernobyl or sendai and other victims of the 2011 tōhoku earthquake and tsunami in this case reliability engineering becomes system safety what is acceptable is determined by the managing authority or customers or the affected communities residual risk is the risk that is left over after all reliability activities have finished and includes the unidentified risk and is therefore not completely quantifiable the complexity of the technical systems such as improvements of design and materials planned inspections fool proof design and backup redundancy decreases risk and increases the cost the risk can be decreased to alara as low as reasonably achievable or alapa as low as practically achievable levels reliability and availability program plan edit implementing a reliability program is not simply a software purchase it is not just a checklist of items that must be completed that ensure one has reliable products and processes a reliability program is a complex learning and knowledge based system unique to one s products and processes it is supported by leadership built on the skills that one develops within a team integrated into business processes and executed by following proven standard work practices 15 a reliability program plan is used to document exactly what best practices tasks methods tools analysis and tests are required for a particular sub system as well as clarify customer requirements for reliability assessment for large scale complex systems the reliability program plan should be a separate document resource determination for manpower and budgets for testing and other tasks is critical for a successful program in general the amount of work required for an effective program for complex systems is large a reliability program plan is essential for achieving high levels of reliability testability maintainability and the resulting system availability and is developed early during system development and refined over the system s life cycle it specifies not only what the reliability engineer does but also the tasks performed by other stakeholders an effective reliability program plan must be approved by top program management which is responsible for the allocation of sufficient resources for its implementation a reliability program plan may also be used to evaluate and improve the availability of a system by the strategy of focusing on increasing testability maintainability and not on reliability improving maintainability is generally easier than improving reliability maintainability estimates repair rates are also generally more accurate however because the uncertainties in the reliability estimates are in most cases very large they are likely to dominate the availability calculation prediction uncertainty problem even when maintainability levels are very high when reliability is not under control more complicated issues may arise like manpower maintainers customer service capability shortages spare part availability logistic delays lack of repair facilities extensive retrofit and complex configuration management costs and others the problem of unreliability may be increased also due to the domino effect of maintenance induced failures after repairs focusing only on maintainability is therefore not enough if failures are prevented none of the other issues are of any importance and therefore reliability is generally regarded as the most important part of availability reliability needs to be evaluated and improved related to both availability and the total cost of ownership tco due to the cost of spare parts maintenance man hours transport costs storage costs part obsolete risks etc but as gm and toyota have belatedly discovered tco also includes the downstream liability costs when reliability calculations have not sufficiently or accurately addressed customers bodily risks often a trade off is needed between the two there might be a maximum ratio between availability and cost of ownership the testability of a system should also be addressed in the plan as this is the link between reliability and maintainability the maintenance strategy can influence the reliability of a system e g by preventive and or predictive maintenance although it can never bring it above the inherent reliability the reliability plan should clearly provide a strategy for availability control whether only availability or also cost of ownership is more important depends on the use of the system for example a system that is a critical link in a production system e g a big oil platform is normally allowed to have a very high cost of ownership if that cost translates to even a minor increase in availability as the unavailability of the platform results in a massive loss of revenue which can easily exceed the high cost of ownership a proper reliability plan should always address ramt analysis in its total context ramt stands for reliability availability maintainability maintenance and testability in the context of the customer s needs reliability requirements edit for any system one of the first tasks of reliability engineering is to adequately specify the reliability and maintainability requirements allocated from the overall availability needs and more importantly derived from proper design failure analysis or preliminary prototype test results clear requirements able to be designed to should constrain the designers from designing particular unreliable items constructions interfaces systems setting only availability reliability testability or maintainability targets e g max failure rates is not appropriate this is a broad misunderstanding about reliability requirements engineering reliability requirements address the system itself including test and assessment requirements and associated tasks and documentation reliability requirements are included in the appropriate system or subsystem requirements specifications test plans and contract statements the creation of proper lower level requirements is critical 16 the provision of only quantitative minimum targets e g mean time between failure mtbf values or failure rates is not sufficient for different reasons one reason is that a full validation related to correctness and verifiability in time of a quantitative reliability allocation requirement spec on lower levels for complex systems can often not be made as a consequence of 1 the fact that the requirements are probabilistic 2 the extremely high level of uncertainties involved for showing compliance with all these probabilistic requirements and because 3 reliability is a function of time and accurate estimates of a probabilistic reliability number per item are available only very late in the project sometimes even after many years of in service use compare this problem with the continuous re balancing of for example lower level system mass requirements in the development of an aircraft which is already often a big undertaking notice that in this case masses do only differ in terms of only some are not a function of time and the data is non probabilistic and available already in cad models in the case of reliability the levels of unreliability failure rates may change with factors of decades multiples of 10 as a result of very minor deviations in design process or anything else 17 the information is often not available without huge uncertainties within the development phase this makes this allocation problem almost impossible to do in a useful practical valid manner that does not result in massive over or under specification a pragmatic approach is therefore needed for example the use of general levels classes of quantitative requirements depending only on severity of failure effects also the validation of results is a far more subjective task than any other type of requirement quantitative reliability parameters in terms of mtbf are by far the most uncertain design parameters in any design furthermore reliability design requirements should drive a system or part design to incorporate features that prevent failures from occurring or limit consequences from failure in the first place not only would it aid in some predictions this effort would keep from distracting the engineering effort into a kind of accounting work a design requirement should be precise enough so that a designer can design to it and can also prove through analysis or testing that the requirement has been achieved and if possible within some a stated confidence any type of reliability requirement should be detailed and could be derived from failure analysis finite element stress and fatigue analysis reliability hazard analysis fta fmea human factor analysis functional hazard analysis etc or any type of reliability testing also requirements are needed for verification tests e g required overload stresses and test time needed to derive these requirements in an effective manner a systems engineering based risk assessment and mitigation logic should be used robust hazard log systems must be created that contain detailed information on why and how systems could or have failed requirements are to be derived and tracked in this way these practical design requirements shall drive the design and not be used only for verification purposes these requirements often design constraints are in this way derived from failure analysis or preliminary tests understanding of this difference compared to only purely quantitative logistic requirement specification e g failure rate mtbf target is paramount in the development of successful complex systems 18 the maintainability requirements address the costs of repairs as well as repair time testability not to be confused with test requirements requirements provide the link between reliability and maintainability and should address detectability of failure modes on a particular system level isolation levels and the creation of diagnostics procedures as indicated above reliability engineers should also address requirements for various reliability tasks and documentation during system development testing production and operation these requirements are generally specified in the contract statement of work and depend on how much leeway the customer wishes to provide to the contractor reliability tasks include various analyses planning and failure reporting task selection depends on the criticality of the system as well as cost a safety critical system may require a formal failure reporting and review process throughout development whereas a non critical system may rely on final test reports the most common reliability program tasks are documented in reliability program standards such as mil std 785 and ieee 1332 failure reporting analysis and corrective action systems are a common approach for product process reliability monitoring reliability culture human errors human factors edit in practice most failures can be traced back to some type of human error for example in management decisions e g in budgeting timing and required tasks systems engineering use studies load cases systems engineering requirement analysis setting systems engineering configuration cont...